Can they use my apple pay or use to buy online when I share with family purchase

No, if you are the family organizer for an Apple Family Sharing group and enable purchase sharing then you may have to pay for new Apple media, app, and subscription purchases made by other members of the family that are not covered by their store credit. They don't get to use your Apple Pay for other purchases.

tt2

Thanks for posting in the Apple Support Communities Junieg,

If you choose to, you can set up Apple Cash Family to allow members who are under 18 access to Apple Pay through the family organizer.

The following page provides additional information, and explains how to get started: Set up and use Apple Cash Family

If you need more help, please contact Apple Support directly: Apple Pay

Cheers!